INVESTIGATION OF ACETYL-CHLORIDE PHOTODISSOCIATION BY PHOTOFRAGMENT IMAGING

The photofragment ion imaging technique is used to measure the velocity distributions of atomic chlorine, methyl. and carbon monoxide fragments generated in the photodissociation of acetyl chloride at 236 nm. The fragments are selectively ionized by (2 + 1) multiphoton ionization and projected onto a two-dimensional position-sensitive detector to obtain speed and angular distributions. The Cl images display anisotropic angular distributions, characteristic of a prompt, impulsive dissociation of the C-Cl bond. A fraction of the CH3CO, produced as a primary photoproduct, subsequently decomposes to form CH3 and CO. The CH3 and CO images are isotropic, suggesting that rotation of the acetyl radical intermediate occurs prior to the secondary dissociation. The internal state distribution of CO is probed using (2 + 1) multiphoton ionization via the B-1 Sigma(+) state near 230 nm. The rotational state distribution of CO extends to S' = 30, while no vibrational excitation is observed. The transition state structure of the CH3CO intermediate, leading to dissociation into CH3 and CO, is computed via ab initio quantum mechanical methods. The barrier for CH3CO dissociation is theoretically predicted at 19.1 kcal/mol at the MP2/cc-pVTZ level of theory. The theoretically predicted dissociation mechanism and barrier agree well with the measured internal state distributions and velocities of the CH3 and CO secondary fragments.